1. Field of the Invention
The present invention relates to gauges for measuring either pressure or temperature down the shaft of an oil or gas well.
2. Summary of the Prior Art
Gauges have been proposed comprising a coiled Bourdon tube containing a fluid, which tube deflects angularly about its longitudinal axis in response to pressure or temperature changes, a stylus carried by an elongate member, the elongate member being coupled to the Bourdon tube so as to deflect angularly in opposite senses in response to coiling and uncoiling of the Bourdon tube, a cylindrical recording chart mounted so as to move in a direction parallel to the rotational axis of the elongate member at a constant rate, the stylus being mounted in contact with the chart so as to mark the chart as the elongate member and stylus move angularly so that a plot of pressure or temperature variation with respect to time is produced on the chart.
Such a pressure gauge is manufactured under code DPG125 by the Assignees of the applicant, Drexel Equipment (U.K.) Limited of Aberdeen, Scotland. Such a pressure gauge is also described in U.S. Pat. No. 3,744,307 to Harper et al issued July 10, 1973 and the tubular chart device which is used in such gauges is described in U.S. Pat. No. 2,590,313 to C. H. W. Hartmann issued Mar. 25, 1952.
A technical problem associated with such gauges is that the uncoiling of the Bourdon tube inevitably results in an axial motion as well as angular deflection of the tube so that, with existing couplings between the stylus shaft and the tube the stylus moves axially as well as angularly. This axial movement is in the direction of the time axis so that it causes a distortion in the resulting chart. The amount of axial motion is small but bearing in mind that the chart may be expected to run for many hours, the distortion caused by axial motion is significant.
The pressure range which needs to be recorded is substantial, from 0 to 10,000 lb/sq.in. (0 to 700 kg./sq.cm) is quite normal and an extension of this range to 20,000 lb/sq.in. (1400 kg./sq.cm) is not unusual. It must also be born in mind that the Bourdon tube will only have a maximum angular deflection from 0.degree. to 310.degree. so that in conjunction with the time scale it will be appreciated that any axial motion of the shaft which transmits motion to the stylus will give rise to an appreciable error in the readings as recorded on the chart. This applies more especially when there is a rapid pressure increase or decrease.
In more detail the conventionl chart is customarily 2" (5.08 cm) by 5" (12.7 cm) in size and the associated clock may be required to run for any time in the range 3 to 144 hours. Generally, either a 24 or 48 hour clock will be used for build-up and draw down of a well bore-hole. An average pressure for the chart will be 7500 psi (527 kg/sq.cm). With parameters such as these, it is not difficult to appreciate that the transmission of axial motion of the Bourdon tube to the stylus will have an adverse effect. FIG. 1 illustrates one specific problem area on a typical chart. Rapid "draw-downs" and "build-ups" of pressure means that the rate of change with time will be critically adversely affected if the axial motion of the Bourdon tube or other sensing device is transmitted to the stylus. The slope of the pressure curve is, in most instances, more important than accuracy, as such, of the pressure (or temperature) reading.
Previously the problem has been at least partially met by special calibration and/or by computer-aided compensation.
Accordingly, an object of the present invention is to solve the technical problem of providing a coupling between the stylus and a Bourdon tube such that the motion between the stylus and the chart is solely a function of elapsed time.